Aircraft surfaces that provide stability and directional control of the aircraft are well known and have been extensively investigated. Fundamental among these are the vertical stabilizer and the rudder. The vertical stabilizer ensures that the nose of the aeroplane is oriented in the direction of flight, and the rudder opposes the yawing moments acting upon the aircraft, when the lateral direction of flight of said aircraft is being controlled.
The size and the operational power of the rudder are determined by, in addition to the aerodynamic requirements for the aircraft, several other factors intrinsic to the aircraft, for example if a fault develops in one of its power plants, and moreover this factor is decisive for the certification of the aircraft by the relevant authorities, for which sufficient control is required for specified speeds and conditions, both on the ground and in flight, for the concrete case of failure of one of the power plants of the aircraft.
The problem of directional control in case of failure of one of the power plants in large aircraft for civil aviation is discussed in various documents, for example in document U.S. Pat. No. 5,375,793. In said document it is stated that, in the majority of cases, it is the pilot who performs the appropriate deflections of the wing control surfaces (in the case when they are justified) during the critical moment of loss of one of the power plants of the aircraft. As described in said document, this manoeuvre is only justified on condition that the aircraft has a tendency to leave its lateral stability, which does not occur in a great many aircraft, so that in these cases the pilot only relies on the maximum deflection of the rudder as an alternative for opposing the yawing moment on the aircraft arising from the failure of one of its power plants.
In this connection, the industry has resorted to increasing the number of aerodynamic control surfaces provided on the wings, for example ailerons, flaps, spoilers and slats, or else has resorted to improving the efficiency of said surfaces. The aim is to improve the lateral-directional dynamic stability of the aircraft by operating said surfaces by means of automatic controls. As a result of this process, during takeoff of an aircraft with engine problems or a cross-wind, which would correspond to situations that would involve a very high yawing moment, with the aircraft speed being low and with very high moments acting on it, due to a fault in a power plant or to the existence of external situations such as a strong cross-wind, the efficiency in yaw of the aerodynamic surfaces is increased automatically by, for example, the application by the pilot of a maximum deflection on the rudder.
The problems that arise in the known solutions of this type relate to the increase in complexity of the structure of the aircraft and of its flight control systems. This gives rise to an increase in weight and increased drag of the aircraft, which leads to an increase in fuel consumption and noise.
The minimum control speed for an aircraft at takeoff is that for which, when a fault occurs in one of its power plants, the pilot is capable of maintaining control of the aircraft by means of deflection of the rudder as a single operation, that is, without this action requiring extraordinary piloting skill. This speed is closely related to the length of the takeoff runway. That is, the vertical stabilizer of an aircraft will be designed so that, at takeoff, if the aeroplane has exceeded its minimum control speed and a power plant fails, its aerodynamic surface in combination with operation of the rudder can absorb the yawing moment acting on the aircraft in consequence of the asymmetrical thrust to which it is submitted, maintaining the directional stability that is necessary for performing a successful takeoff manoeuvre. Below this minimum control speed the aeroplane must fulfil the requirement of being capable of performing a successful braking manoeuvre, i.e. within the length of the runway and complying with all standards relating to passenger safety.
Taking the foregoing into account, it is desirable for the minimum control speed to be as low as possible, so that the aeroplane can operate on shorter runways. To have lower minimum control speeds implies that the surface area of the vertical stabilizer and the surface area and power of the rudder should be greater, which means a penalty in weight and drag, as well as increasing the costs of manufacture, the resultant weight of the structure and the fuel cost in flight. The present invention overcomes these drawbacks, so that it provides greater rudder area in case of engine failure while maintaining the minimum area for the requirements of airworthiness of the aircraft in other flight conditions and regimes, and therefore without producing a penalty in increased drag and consequent increase in fuel cost and of efficiency in the thrust of the power plants.
As has already been explained, several inventions have been developed that aim to reduce the size of the vertical stabilizer and maintain the characteristics of directional control of the aircraft by increasing the aerodynamic control surfaces of the wings, as for example in documents WO 03/016133 A1, US 2007/0102587, U.S. Pat. No. 4,132,375 or in the aforementioned document U.S. Pat. No. 5,375,793. Solutions of this type increase the complexity of the control systems of the aeroplane and limit the crew's capacity for reaction and piloting to the maximum deflection of the rudder, an inadequate manoeuvre unless it is coupled with activation of the systems described. Conversely, the use of spoilers or ailerons (aerodynamic surfaces of the wings) can create a rolling moment on the aircraft, which will be controlled by the use of other surfaces, which will give rise either to an unnecessary increase in work load for the pilot, or to greater complexity of the automatic systems for flight control. That is, increase of the aerodynamic control surfaces on the wing leads to increased drag and therefore reduces the thrust capability of the engines, i.e. the aeroplane's ability to accelerate at a critical moment when this characteristic can prove decisive. Another drawback of having more aerodynamic surfaces on the aircraft is that their actuation results in a considerable increase in noise. Problems of the type described can increase exponentially if, as mentioned in document US 2006/0284022, we extend the use of these aerodynamic surfaces to other elements of the aircraft, such as the fuselage or the tail unit.
Another advantage of the present invention relative to other existing solutions is its simplicity. There are many inventions (U.S. Pat. No. 2,643,833, U.S. Pat. No. 5,681,010, U.S. Pat. No. 2,941,752) that claim the concept of adjusting the area of the tail unit in relation to the flight phase of the aircraft, but they add a great quantity of mechanical elements to the structure, which leads to increased weight of the unit and therefore poorer energy efficiency, as well as various drawbacks, such as a penalty in the time for centring the aircraft.
The present invention aims to solve the drawbacks mentioned above.